Control of the timing of DNA synthesis and nuclear division is essential for maintaining a state of balanced growth in proliferating cells. There is considerable evidence in bacterial and higher cells that the initiation of events in the DNA-division cycle is triggered by a cell size monitor. It is proposed to study the regulatory system controlling synchronous nuclear division in plasmodia of the acellular slime mold Physarum polycephalum. Our aim is to learn more about the molecular mechanism which regulates the cellular DNA to mass ratio. Our procedures include perturbation of the normal cell cycle with metabolic inhibitors, resolution of cell cycle dependent proteins by two dimensional gel electrophoresis and theoretical analysis of models of possible control systems.